Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a drill string. The well is drilled so that it penetrates the subsurface formations containing the trapped materials and the materials can be recovered.
The drilling operations are controlled by an operator at the surface. The drill string is rotated at a desired rate by a rotary table, or top drive, at the surface, and the operator controls the weight-on-bit and other operating parameters of the drilling process. At the bottom end of the drill string is a “bottom hole assembly” (“BHA”). The BHA includes the drill bit along with sensors, control mechanisms, and the required circuitry. A typical BHA includes sensors that measure various properties of the formation and of the fluid that is contained in the formation, as well as the operating conditions of the drill bit and other downhole equipment.
Another aspect of drilling and well control relates to the drilling fluid, called “mud.” The mud is a fluid that is pumped from the surface to the drill bit by way of the drill string. The mud serves to cool and lubricate the drill bit, and it carries the drill cuttings back to the surface. The density of the mud is carefully controlled to maintain the hydrostatic pressure in the borehole at desired levels.
In order for the driller at the surface to be aware of the downhole conditions and for the driller to be able to control the drill bit, communication between the BHA and the surface is required. One common method of communication is called “mud pulse telemetry.” Mud pulse telemetry is a method of sending signals by creating pressure and/or flow rate pulses in the mud. These pulses may be detected by sensors at the receiving location. For example, a telemetry signal may be sent from the tool to the surface as pressure pulses in the mud flow downwardly through the drill string. The pressure pulses may be detected and interpreted at the surface.
A typical downhole mud pulse telemetry tool includes a restrictor (or rotor) and a stator. The restrictor rotates with respect to the stator to vary the cross sectional area of the mud flow passage through the mud-pulse telemetry tool. Because the mud is pumped at the surface using positive displacement pumps, the flow rate of the mud will remain relatively constant. By using the restrictor and the stator to restrict the area of flow, the pressure of the mud flowing in the drill string will increase. Correspondingly, by manipulating the restrictor and stator to increase the flow area, the pressure in the drill string will decrease. Selective operation of the restrictor and stator may create specific pressure pulses in the drill string that may be sensed and interpreted at the surface.
Typically, a motor and gear train is coupled to the restrictor so that the restrictor may be selectively manipulated. In many tools, the motor/gear train is coupled directly to the restrictor. In these tools, rotary fluid seals are required to prevent the drilling fluid from contaminating the lubricant in the motor and gear train. Because of the abrasive nature of the mud, these seals are prone to failure.
One possible method for improving the reliability of a downhole mud pulse telemetry tool is to use a magnetic coupling between the motor/gear train and the restrictor. A magnetic coupling does not require a rotary seal. It enables the motor/gear train to be completely enclosed so that the mud cannot contaminate the lubricant inside the motor/gear train.
One significant problem with downhole mud pulse telemetry tools is that they occasionally become jammed with particles from the drilling mud. The occurrences of jamming increase when particles are added to the mud to correct problems such as lost circulation. The particles are used form a barrier against the borehole wall to seal the inside of the borehole from the formation so that mud will not flow into the formation. One of the side effects is that the particles become lodged between the blades of the restrictor and the stator, preventing relative rotation between them.
Techniques have been developed that prevent jamming in downhole mud pulse tools. A typical prior art anti-jam technique is to apply a much higher torque to the restrictor to cut through the jamming material. Examples of anti-jam techniques are described in U.S. Pat. No. 6,219,301, assigned to the assignee of the present invention, and U.S. application No. 2004/0069535 assigned to Baker Hughes.
Despite these advances in anti-jam technology, there remains a need for anti-jam techniques capable of operating reliably without requiring higher torque. It is further desirable that such a system be operable in devices where the restrictor is magnetically coupled to the motor/gear train. The size of the magnetic coupling may depend on the torque that the magnetic coupling is required to transmit. Moreover, the torque required to cut through particles jammed in the restrictor blades may require a magnetic coupling that is undesirably long, unstable and/or unreliable.
What is needed, therefore, is a telemetry tool with an anti-jam feature that does not require high torque for shearing lodged particles.